Due to the increasing energy demand, offshore oil and gas production is moving into deeper waters. For providing an efficient and secure production of hydrocarbons from a subsea well, processing facilities are being installed at the ocean floor. Such subsea installations may include a range of components, including pumps, compressors, and the like as well as a power grid for providing such components with electric power. The power grid may, for example, include a subsea transformer, subsea switchgear, and subsea variable speed drives (VSDs). Such components of a subsea installation may be installed at water depths of 3,000 meters or more, so that they are exposed to pressures up to or even in excess of 300 bars. To protect such components from the corrosive seawater and to handle the high pressures prevailing in such subsea environment, these components are provided with subsea enclosures.
The construction of such subsea enclosures is technically challenging. Two solutions are proposed for dealing with these high pressures. A pressure resistant enclosure may be provided, which has a close to atmospheric internal pressure, enabling the use of conventional electric and electronic components. Such enclosures need to have relatively thick walls and are thus bulky and heavy, since they have to withstand high differential pressures.
Another solution is the use of pressurized (or pressure compensated) enclosures, which include a volume/pressure compensator that balances the pressure in the enclosure to the pressure prevailing in the ambient seawater. The pressure compensated enclosure may be filled with a liquid, and components operated inside the pressure compensated enclosure are made to be operable under high pressures. The pressure/volume compensator compensates variations in the volume of the liquid filling the enclosure, which may occur due to variations in ambient pressure and/or in temperature. Temperature changes may be caused by deployment at the subsea location and by internal heating, e.g., due to electric losses.
Pressure compensators may include metal bellows, rubber bellows, pistons, or the like. As an example, the document WO 2010/034880 A1 discloses a pressure compensator that has a first bellows chamber surrounded by a second bellows chamber, the second bellows chamber forming a closed intermediate space around the first bellows chamber. A double barrier against the ingress of seawater is thus obtained.
Furthermore, the document WO 2011/088840 A1 discloses a pressure compensation system that achieves a double barrier against the ingress of seawater.
Such type of double barrier may increase the safety and reliability of the pressure compensator only insignificantly. A failure of the pressure compensator may be caused by fatigue of the material of the bellows part, e.g., by the formation of cracks and thus leaks in the bellows part of the pressure compensator. As an example, through such crevice in the outer bellows portion, seawater may leak into the intermediate chamber, where it is stopped by the inner bellows part. Even so, both bellows parts undergo the same compression/decompression cycles, and are accordingly experiencing the same stress and fatigue. Both bellows portions are thus likely to fail within a relatively short period of time. Accordingly, the lifetime and thus the safety and reliability of such double barrier pressure compensator are not extended significantly by the second barrier.
U.S. Patent Publication No. 2013/0167962 A1 discloses a configuration that makes use of a particular arrangement of two bellows, thus increasing the compensation volume and keeping the dead volume small. A single barrier configuration having an improved reliability may thus be achieved.
Nevertheless, it is desirable to further increase the reliability of a pressure compensator for a subsea device. It is desirable to both improve the protection against ingress of seawater, and furthermore increase the lifetime over which such pressure compensator may be operated.